As a U.S. Navy carrier battle group entered the Persian Gulf, it came under surprise attack by adversaries launching missiles from commercial ships and radio-silent aircraft that quickly overwhelmed its missile defense systems. Nineteen U.S. ships, including the aircraft carrier, were destroyed and sunk within 10 minutes.

Fortunately for U.S. forces, this scenario was only a simulation in a massive, $250 million war game named Millennial Challenge 2002. After the unexpected and humbling “loss” in July 2002, military officials at Joint Forces Command in Norfolk paused the war game, “refloated” the ships and restarted the exercise. They also imposed limits on enemy tactics. After the restart, the U.S. forces defeated their adversaries in a more conventionally fought simulation.

An after-action report of the exercise - which has remained secret for over 20 years - reveals that the surprise defeat triggered internal warnings that the U.S. military was vulnerable to low-tech warfare, foreshadowing the very challenges the United States would face in the 2003 invasion of Iraq and other conflicts since then. The Post recently obtained the report in response to a Mandatory Declassification Review request (MDR).

The after-action report was written by retired Marine Lt. Gen. Paul Van Riper, who commanded the enemy forces during the war game. “We looked for their weakness and put our strength against it,” Van Riper told The Post.

I first learned of this classified document by reading a 2002 Army Times article in which Van Riper criticized Millennium Challenge 2002 as “rigged” and mentioned “a 20-page report” that he had submitted to his superiors.

With this information, I filed the declassification request to the Office of the Secretary of Defense in 2013. At the time, I was working for the National Security Archive, a nonprofit based at George Washington University that fights to make government records of historical significance public.

Eleven years later, I got my answer - an email explaining that the record I had requested had been declassified in part. The released report demonstrates why it is important to push back against government secrecy: Unearthed records may reveal to the public critical information omitted from official government narratives.

Van Riper’s write-up contradicts portions of an official 752-page final report on Millennium Challenge 2002 released by the military more than a decade ago that called the war game a “major milestone” and described the loss of an entire carrier group as only “moderately unsuccessful.”

It also reveals the restrictions the U.S. military eventually imposed on the enemy and Van Riper’s conclusion that by limiting his tactics, the U.S. military ensured victory and de-emphasized the critical vulnerabilities he had identified.

Mandatory Declassification Review requests are similar to, but distinct from, Freedom of Information Act (FOIA) requests. Unlike FOIA, which was a law passed by Congress, the Mandatory Declassification Review provision is governed by an executive order.

The requirements for submitting a MDR request are defined by President Barack Obama’s Executive Order 13526.

Because MDR requests are frequently reviewed by government declassification experts who hold high-level security clearances, requesters usually have a better shot at winning the release of classified records than with FOIA requests, which are often reviewed by officials lacking such expertise.

But like FOIA requests, MDR delays can be frustrating. This request was reviewed for release by five entities: Office of the Secretary of Defense, the Joint Chiefs of Staff, the Central Intelligence Agency, U.S. Central Command, and the U.S. Army. And the released record is moderately redacted, which - of course - I have appealed.

The Defense Department press office did not respond to multiple Washington Post queries as to why it took more than 11 years to process the request.

Millennium Challenge 2002 was touted at the time by the Defense Department as “the largest-ever joint military experiment conducted by the United States.” It took two years to develop, involved more than 13,500 participants and unfolded over three weeks in July and August 2002.

Millennium Challenge 2002 resembled a much more complicated version of recreational, countertop war games: Participants simulated the conflict at 17 sites as wargamers conducted maneuvers against each other on a military computer network.

Adjudicators used computer models to determine the outcome of their attacks and other operations.

The simulated conflict also was combined with live training of troops and equipment at nine locations in the United States. Joint Forces Command (JFCOM) Commander Gen. Buck Kernan, who oversaw the war game, in public remarks at the time described the opposing forces as “very very, determined … this is free play. [Van Riper] has the opportunity to win here.”

In many ways, Millennium Challenge 2002 was a rehearsal for the 2003 Iraq War. After the game was conducted, then-Defense Secretary Donald H. Rumsfeld sent a note to Kernan asking him to write a report explaining “what you think you learned from Millennium Challenge that we ought to apply to Iraq.”

Kernan did not respond to requests for comment. But the official report released in 2012 by Joint Forces Command about Millennium Challenge 2002 said that the exercise “provided an analogous complex situation” to the Iraq War.

In the exercise, Van Riper played the “Major General” of a country resembling Iraq or Iran that “possessed natural resources critical to the world community.”

His report notes that he used a strategy of ambiguity, asymmetry and denial of territory to have his forces, known as “Red,” defeat the superior U.S. military. He wrote that because the U.S. forces, designated “Blue,” appeared determined to go to war, he “saw no option except to strike Blue first.”

To plan his attack, Van Riper wrote that he “employed a command and control methodology specifically designed to thwart” American technological advantages, including the ability to intercept electronic and phone communications. He relied on couriers to relay sensitive messages and communicated to aircraft with lanterns to avoid radio chatter.

After his surprise attack simulated the destruction of the carrier group, the atmosphere at Norfolk command, where Van Riper led his team, was “shock,” he told The Post in an interview. “It was just quiet. It never happened in an exercise before. ... I don’t think [Joint Forces Command] knew what to do.”

Van Riper wanted to continue to attack U.S. forces, pressing forward with his asymmetric advantage, his report notes. Instead, a war game adjudicator determined Van Riper’s successful attack “wouldn’t have happened” in real warfare and ruled that all but four of the virtual U.S. ships would be “refloated” and the war game would continue, according to his report.

In an interview, Micah Zenko, an expert on war games and author of the book “Red Team: How to Succeed By Thinking Like the Enemy,” said that Van Riper may not have taken into account the full picture in his desire to keep playing. He said that U.S. Joint Forces Command was charged with “scoping, designing, and running, an extremely complex and elaborate war game ... if ‘refloating’ the Blue’s maritime forces is required to do that, they will do so.”

Zenko noted that the war game also involved real activity. Of the forces participating, 20 percent were practicing live fire exercises on U.S. bases, including Nellis Air Force base, Nev., and Fort Irwin, Calif. If the U.S. carrier group was not refloated, these live exercises could not have been realistically conducted.

Still, Zenko praised Van Riper’s report for being “devastating in the specific critiques he witnessed.”

Van Riper, in his report, noted that the ultimate significance of the elimination of realistic war gaming in Millennium Challenge 2002 was that it created an exercise for which the “result was preordained.” As such, he believed the exercise was not a useful test of the U.S. military’s ability to invade a hostile nation.

After the U.S. carrier group was “refloated,” other restrictions were imposed on Van Riper, he noted in his report. His forces could not initiate combat, but U.S. forces could. Van Riper’s forces were also forbidden from using chemical weapons against the United States, which he considered his country’s “most significant” asymmetric military strength.

Having lost the independence he believed was integral to the war game, Van Riper stepped down as commander of opposition forces but continued to monitor the war game as an adviser, he told The Post.

At the conclusion of the war game, he wrote that the results of Millennium Challenge 2002 “need to be considered in light of the fact that the [United States] commander did not operate against a ‘thinking and adaptive’ enemy who ‘could win’” after the American fleet was refloated. “Not having a ‘thinking and adaptive’ enemy operating against [United States forces] will have a very significant impact on the assessment” of U.S. warfighting concepts, he wrote.

Van Riper believes that the public should “absolutely” have been able to read this report decades ago. He says that with the report’s current redactions, “they could have declassified it the next day,” in 2002.

“The real sad thing,” he said, “is some of the things that we learned were never shared.”

‘Craig Whitlock, Dan Lamothe and Aaron Schaffer contributed to this report.